Mining soluble minerals using passageway formed by fracturing



MlNlNG SGLUBLE NERAS USING PASSAGE- WAY FRMED BY FPACTURING Alden W. Hanson, Midland, Mich.

Application h/larch i9, i955, Serial No. 572,264

2l Claims. (Cl. 262-3) lhis invention relates to mining soluble minerals.

Such minerals have been mined commercially by pumping a solvent into the vein or bed in which the mineral is found and returning the saturated solvent to the surface of the earth and extracting the mineral. Commercial practice includes the use of concentric supply and withdrawal pipes in a single well. The use of separate injection and production wells spaced at appropriate distances has also been suggested.

The present invention contemplates the use of massive solvent flow to increase the rate of yield. For this purpose, it is proposed to use separate injection and production wells. However, particularly when the wells are located at some distance apart, and when the mineral is highly soluble (as, for example, the mining of sodium chloride with water) it becomes diicult to open up an adequate passage for massive iiow between the injection and the production wells because of the very considerations which would suggest that this should be a simple matter. The very fact that the mineral is extremely soluble results in almost immediate saturation. Thus, before the injected liquid has progressed any distance at all from the injection well it becomes completely saturated and incapable of opening up a passage by dissolving any further mineral.

ln accordance with the present invention, the mineral stratum between the injection and production wells is rst channeled. This is desirably accompalished by the use of extremely high pressure in the injection and/or production well to fracture the stratum and form a crack or fissure which extends from one well to the other. The fissure is then enlarged to accommodate massive solvent now, withL the result that mineral is brought to the surface at a rate in excess f anything heretofore thought possible.

Enlargement of the ssure is a serious problem. While the fissure may be held open by maintaining high pressure on the injected solvent (using baci; pressure at the production well if necessary) the cross section of the fissure will not ordinarily be suliiciently enlarged thereby to accommodate massive solvent flow.

ln most instances, ancillary procedures must be employed to produce the desired communicating channel. The need for some ancillary procedure becomes increasingly great as the two wells are more remote from each other. lt also becomes great in the event that there is a cavern of substantial size at the base of the injection well, since a large cavern increases the tendency of the injected solvent to become saturated before it has an opportunity to enlarge the fissure between the wells.

Accordingly, an important feature f the present nvention is to inhibit the dissolving of the mineral into the solvent until the solvent reaches a predetermined point where its action in picking up mineral will enlarge the fissure. To this end, the solvent may initially comprise or carry any one of a number of inhibiting factors which, as hereinafter described, will maintain the solvent in an unsaturated condition until it has reached and pene- Patented Sept. 2, 1958 trated the ssure, thereupon achieving increased capacity for picking up minerals from portions of the i'issure which are progressively more remote from the injection well until nally the ssure has been enlarged to a degree sulicient to permit of continued massive flow.

In the drawings:

Fig. l is a diagrammatic view taken through the earth and a mineral stratum extending between an injection and production well.

Fig. 2 is an enlarged diagrammatic cross section taken through earth strata and illustrating the formation of a cavern at the bottom of the injection well and the iracturing of the stratum laterally therefrom.

Fig. 3 is an enlarged diagramatic cross section taken through a cavern at the bottom of an injection well and illustrating the use of rubber or latex balloons to isolate incoming solvent from previously saturated solvent present in the cavern.

Fig. 4 is a view similar to Fig. 3 and showing the cavern wall coated with a barrier to isolate incoming solvent from contact with the cavern wall.

The method of my invention is applicable to the mining with water of most soluble minerals, such as sodium nitrate (NaNOB), potassium chloride (KCI), and sodium chloride (NaCl). lt also applies to some of the alkaline earth metal salts. illustrative examples are Epsom salt (lVlgSO417H2O), kiersite (MgSOr-HZO), calcium chloride (CaClz). Many water soluble minerals are in the form of double salts, e. g., kainite, sylvanite, carnallite. lt is to be understood, however, that my invention in its broad aspects is not limited to a specic mineral or a specific solvent.

In the diagrammatic showing in the drawing, an injection well 5 and a production well 6 have been drilled into the earth '7 until they reach a mineral bearing stratum 8 which may be, for example, a vein of salt (sodium chloride). Such strata may be disposed at varying depths and are commonly found anywhere from 50 to 5,000 feet below the earths surface. The distance between the wells 5 and 6 may vary widely. Depending on circumstances, the wells may be as close as 60 feet and as far apart as 1,000 feet or more.

In order to fracture the stratum S between the two wells, it is desirable to first form caverns 9 and 10 at the bottoms of the respective wells. The Caverns may be formed in the conventional manner by pumping solvent down to the cavern through one passage of concentric well pipes 13, 14, and drawing the saturated solvent up through another passage therein.

In the practice of my invention the caverns are desirably made no longer than is necessary to provide a sutlicieut area over which solvents supplied under pressure can act to lift the overburden. The solvent, or a conventional fracturing mixture, may then be introduced into one or both of the Wells and Caverns and pressurized to fracture the stratum between the injection and production wells. The crack or ssure resulting from the fracturing step is diagrammatically illustrated by reference character 11. The stratum is fractured after enough pressure is developed in the respective caverns 9 and l0 to over- -come the overburdening pressure of the earth above the level of the stratum and to overcome the cohesive forces ibinding the structure of the stratum.

The crack 11 thus formed will follow the line of least resistance between the injection well and production well and will take a course dictated by the nature of the formation through which it passes. For a sodium chloride stratum, I find that it ordinarily requires about 1300 p. s. i. (over and above the overburdening pressure) to fracture the stratum. For a well of moderate depth the overburdening pressure may be as high as 8,000 or 9,000 p. s. i. For the same type stratum l iind that about 800 p. s. 1'.

by providing production well 6 with ow restricting means such-'as the throttle valve 4 while solvent is forced into well 5 by pump 3, may be employed.

Enlargement of the ssure 11 depends primarily upon exposing .its walls to relatively unsaturated solvent. If untreated fresh solvent is pumped down well 5, however, the solvent becomes completely saturated by exposure to the walls of cavern 9 before the solvent reaches the fissure 11. Such saturated solvent as reaches the ssure is inetfective to dissolve mineral from the fissure walls.

An important feature of my method is to inhibit any substantial degree of saturation of the solvent before it reaches the ssure constriction. This may be accomplished byfollowing one or more of the following somewhat diiferent procedures, all of which, however, are intended to isolate the solvent from contact with the cavern wall andsaturated solvent contained therein until the solvent reaches the fissure.

(1) Compressed air or yother gas maybe pumped into the cavern above the level of the ssure to reduce the contact area of solventin the cavern with the cavern wall.

(2)y A liquid natural or synthetic rubber, desirably in emulsion, such Vas a latex, may be added to the solvent to coat the wall of the cavern, as shown at 15 in Fig. 4, and thus isolate the mineral wall from contact with the solvent.

(3) The solvent may be pumped into the well as the inner phase of an unstable emulsion initiallyprotected from contact with the mineral well or mineral saturated solventby the outer phase of the emulsion. Examples of substances suitable for the outer phase of the emulsion are oil and inorganic and organic compounds. Examples of emulsiiiers are: sodium oleate, sodium stearate.v The organic `emulsion may be a latex of a chlorinated hydrocarbon of the formula (C2H2Cl2)n. Organic gels may be used such as those of cellulose derivatives, e. g., cellulose ethers (methyl cellulose), gums or starches, preferably coupled with bacteria and enzymes. v

The outer phase of the emulsion will prevent contact of the inner solvent phase with the mineral wall orsaturated solvent. However, the unstable emulsions willfultimately break down, preferably at a point in the flow of the liquid where it is desirable that the solvent lbe released for action on the fissure constriction. As the emulsion breaks down its outer phase may precipitate in the form of a curd which will deposit on the mineral wall in the form of a coating to isolate Vthe solvent from the mineral.

In this connection the proportion of :solvent to emulsi` l er should be as high as possible as it is desirable that the outer phase of the emulsion precipitate only on the mineral wall and not be carried along with the solvent to contaminate the product. For example, where oil `is the outer phase of the emulsion and water the inner phase, the water may constitute about ninety percent of 'the Y total.

VAs the ice particles melt, fresh water will be released for direct contact with the fissure wall.

(5) The ysolvent may be introduced into the injection well at a temperature different from the temperature of the mineral stratum. As the solvent flows through the fissure it will exchange heat with the earth and tend to come into temperature equilibrium therewith. In the Course of heat exchange. the dissolving capacity of the solventwill increase to dissolve mineral from the issure wall at points therealong which are remote from the point of solvent introduction. For example, for minerals Which dissolve more readily as the solvent temperature Y increases, such as sodium chloride in water, the temperature of the water may be initially lowered to the point where it remains a liquid but has relatively little dissolving power compared to its dissolving power at higher temperatures. As the cold water is pumped through the fissure it will take heat from the earth and rise in temperature to progressively increase its dissolving power as it ows. For minerals which dissolve more readily as the solvent Vtemperature decreases, such as sulphate salts in water, hot water may be pumped through the fissure. As the water gives up heat to the earth its temperture will gall to progressively increase its dissolving power as it ows.

(6) The solvent may be introduced into the injection well in the physical form of Vapor which does not dissolve mineral from the fissure wall until it condenses. Thus, high pressure steam may be injected into the injectionwell 5 to condense on the walls of fissure 11 and dissolve mineral therefrom. The pressure of the steam may be maintained high enough to hold the fissure open, or conventional propping agents may be initially introduced into the fissure through which the steam may readily pass to more remoe portions of the fissure.V This method has additional advantages in that the cavern 9 may be kept free of liquid solvent, thus to reduce dissolution of mineral from the cavern wall. The steam may be injected through pipe 13` (see Fig. 2) and condensed solvent removed via pipe 14 from the bottom of the cavern which functions as a sump.

(7) Another method of isolating the incoming solvent from contact with the cavern wall and its saturated solvent content is to add a film-forming elastomer, such as latex emulsion, to the incoming solvent. The yelastomer will cohere and form an expanding envelope or balloon 12 (see Fig. 3) at the bottom of the injection well shaft within which the solvent continues to flow. 4If the envelope breaks, as at 16,.another envelope 17 will simply form beyond the break. In practice there may be a succession of envelopes 12,17 and 18 before the fissure 11 is ultimately reached. v

The ssure is the only outlet from the cavern. Accordingly, the direction of solvent llow from the injection Well casing will be toward the fissure and the envelope tube will normally form itselftoward the fissure.

InV practice I may pump fresh solventv down the injection well 5 for a short interval, for example fifteen minutes, and then pump a mixture of solvent and elastomer for an equal interval, alternating `'fresh solvent with treated solvent. By testing the concentration ofmineral 1n the output well 6, I can detect when the envelope tube has bridged the gap between the bottom of the well bore 5 and the fissure inlet 11.

As before indicated the initial pumping rate through the fissure 11 is extremely low, perhaps as low as' one quart of liquid solvent per minute. However, as soon as the crevice 11 has been enlarged by dissolution of its wall by the solvent, I may increase the pumping rate enormously. In practice I may attain a Vpumping rate of 2,000 gallons of solvent per minute.v At this extremely high'pumping rate the solvent is circulated so rapidly through the fissure that it does not have time to becomesaturated before it reaches'anyrissure constriction. This ow of solvent will continuously enlarge the ssure and make possible even higher pumping rates. After the fissure has been enlarged beyond three inches in diameter, the foregoing treatment of the solvent may be discontinued and fresh solvent simply pumped through the well system.

I claim:

1. A method of mining soluble minerals from a stratum between an injection and production well system, said method comprising the steps of fracturing the stratum between said wells, pumping a solvent through said well system and through the stratum fissure formed by the fracturing step and inhibiting the dissolution of the mineral by the solvent until the solvent reaches a fissure constriction.

2. The method of claim 1 in combination with the further step subsequent to initial enlargement of the fissure in which the dissolution of the mineral by the solvent is inhibited by pumping the solvent at such a high velocity that it reaches a fissure constriction before it is saturated with mineral.

3. The method of claim 1 in which the dissolution of the mineral by the solvent is inhibited by coating the stratum wall about the path of the solvent in advance of the fissure constriction with a substance which isolates the solvent from the mineral until the solvent has passed beyond the coated area of the stratum.

4. The method of claim l in which the dissolution of the mineral by the solvent is inhibited by suspending the solvent as the inner phase of an unstable emulsion and breaking down the emulsion to release the solvent at the fissure constriction.

5. The method of claim 1 in which the dissolution of the mineral by the solvent is inhibited by solidifying particles of solvent at temperatures below the freezing point thereof and thawing such particles in the course of their movement through the fissure to release the solvent in liquid form at the fissure constriction.

6. The method of claim 1 in which the dissolution of the mineral by the solvent is inhibited by introducing the solvent at a temperature different from earth stratum temperature and at which temperature said solvent has less dissolving power than its dissolving power at earth stratum temperature, the heat exchange between the injected solvent and the earth stratum bringing the solvent temperature toward earth stratum temperature to progressively increase the dissolving power of the solvent as it iiows through the fissure.

7. The method of claim 1 in which the dissolution of the mineral by the solvent is inhibited by owing with said solvent a film-forming elastomer which will inliate at the bottom of the injection well and isolate solvent within said film from mineral outside said lm.

8. The method of claim 7 in which successive increments of elastomer are intermittently added to the solvent to form successive iniiated lms as previously formed lms rupture.

9. A method of mining soluble minerals from a stratum between an injection and production well system, said method comprising the steps of forming a pressure multiplying cavern at the bottom of the injection well, subjecting said cavern to a pressure in excess of the overburdening pressure plus the fracturing pressure of the stratum whereby to fracture the stratum and form a fissure between the injection and production wells, pumping a mineral solvent through the well system and through the stratum ssure, and inhibiting the commingling of the incoming solvent with mineral saturated solvent present in the cavern until the solvent reaches a ssure constriction.

10. A method of isolating fresh solvent from mineral saturated solvent in a cavern at the bottom of a well having a ssure outlet, said method comprising adding a film-forming elastomer to the fresh solvent and pumping the resultant mixture down the well to displace a portion of the cavern contact through said fissure outlet, said film-forming elastomer cohering at the bottom of the Well to form an expanding envelope separating its fresh solvent content from the saturated mineral content of the cavern.

1l. The method of claim l0 in which pumping is con- Vtinueduntil said envelope ruptures, and the further steps of continuing to add film-forming elastomer to said solvent to cohere a second expanding envelope to the ruptured margins of the envelope first mentioned and in like manner form a chain of envelopes until the fissure outlet is reached whereby to seal a passage for said fresh solvent from said well bottom to said fissure outlet.

12. A method of treating mineral solvent to temporarily inhibit its action on the first increments of mineral disposed along a path, said method comprising isolating said solvent from contact with said first increments of mineral by suspending said solvent as the inner phase of an unstable emulsion which breaks down beyond said rst increments of mineral to release fresh solvent for action on subsequent increments of mineral on said path.

13. A method of inhibiting exposure of fresh solvent to the wall of a mineral cavern at the bottom of a well comprising coating said mineral wall to bar contact of the solvent with the wall.

14. A method of treating fresh solvent to temporarily inhibit its action on the rst increments of mineral disposed along a path, said method comprising freezing particles of said solvent to a temperature at which said particle Will not thaw until said first increments are bypassed thereby to release fresh solvent for action on subsequent increments of mineral on said path.

15. A method of treating fresh solvent to temporarily inhibit its action on the first increments of mineral disposed along a path, said method comprising introducing said solvent at a temperature different from mineral temperature and at which temperature said solvent has less dissolving power than its dissolving power at mineral temperature, and exchanging heat between the solvent and mineral to tend to bring the solvent and mineral into temperature equalibrium to progressively increase the dissolving power of the solvent as it flows through subsequent increments of mineral on said path.

16. The method of claim l5 in which the temperature of the injected solvent is lowered to a point below the mineral temperature for a mineral having a normal solubility characteristic.

17. The method of claim 15 in which the temperature of the injected solvent is raised to a point above the mineral temperature for a mineral having a reverse solubility characteristic.

18. A method of treating fresh solvent to temporarily inhibit its action on the iirst increments of mineral disposed along a path, said method comprising converting said solvent into vapor at a temperature different from the temperature of the mineral and passing said vapor along said path to bypass said rst increments, exchanging heat between said vapor and mineral to condense sai` vapor to liquid to thereby release fresh solvent for action on subsequent increments of mineral on said path.

19. A method of inhibiting exposure of fresh solvent to a lirst portion of the wall of a channel formed through a mineral stratum and exposing said solvent to a subsequent portion thereof, said method comprising vaporizing said solvent and passing it in the form of a gas through said rst portion and condensing said vapor on the wall of said second portion.

20. A method of mining soluble minerals from a stratum between injection and production wells, said method comprising the steps of pumping solvent into and from the respective wells individually until caverns have been produced in the stratum; subjecting liquid in at least one of the wells to sufficient pressure to effect a stratum fracture between said wells, pumping solvent into the injection well under sutiicient pressure to maintain the fracture while restricting iiow from the producing well Y Wells has dissolved sufficient mineral torprovide an opening between said Wells independently of the fracture, and thereupon relieving the fracture-maintaining pressure while eiecting massive -flow `of solvent between `saidwells to dissolve. additional mineral along the passage thus opened.

2l. The method recited in claim 20 in combination with the further step of temporarily inhibiting the dissolving of mineral by said solvent for a substantial period following the introduction of the solvent from the injection well into the stratum. Y

References Citedin the tile of this-patent UNITED STATES PATENTS FOREIGN PATEN'IS,` l Great Britain Oct.'` 13, 1954 

1. A METHOD OF MINING SOLUBLE MINERALS FROM A STRATUM BETWEEN AN INJECTION AND PRODUCTION WELL SYSTEM, SAID METHOD COMPRISING THE STEPS OF FRACTURING THE STRATUM BETWEEN SAID WELL, PUMPING A SOVLENTHROUGH SAID WELL SYSTEM AND THROUGH THE STRATUM FISSURE FORMED BY THE FRACTURING STEP AND INHIBITING THE DISSOLUTION OF THE MINERAL BY THE SOLVENT UNTIL THE SOLVENT REACHED A FISSURE CONSTRICTION.
 20. A METHOD OF MINING SOLUBLE MINERALS FROM A STRATUM BETWEEN INJECTION SAID PRODUCTION WELLS, SAID METHOD COMPRISING THE STEPS OF PUMPING SOLVENT INTO SAID FROM THE RESPECTIVE WELLS INDIVIDUALLY UNTIL CAVERNS HAVE BEEN PRODUCED IN THE STRATUM; SUBJECTING LIQUID IN AT LEAST ONE OF THE WELL TO SUFFUCIENT PRESSURE TO EFFECT A STRATUM FRACTURE WHILE RESTRICTING FLOW FROM THE PROVIDING WELL JECTION WELL UNDER SUFFICIENT PRESSURE TO MAINTAIN THE FRACTURE WHILE RESTRICTING FLOW FROM THE PRODUCING WELL SUFFICENTLY TO MAINTAIN BACK PRESSURE REQUIRED TO KEEP THE FRACTURE OPEN UNTIL THE FLOW OF SOLVENT BETWEEN SAID WELL HAS DISSOLVED SUFFICIENT MINERAL TO PROVIDE AN OPENING BETWEEN SAID WELLS INDEPENDENTLY OF THE FRACTURE, AND THEREUPON RELIEVING THE FRACTURE-MAINTINING PRESSURE WHILE EFFECTING MASSIVE FLOW OF SOLVENT BETWEEN SAID WELLS TO DISSOLVE ADDITIONAL MINERAL ALONG THE PASSAGE THUS OPENED. 